1. Field of the Invention
The present invention relates to composite materials. More particularly, this invention is directed to the fabrication of fiber-reinforced titanium alloy matrix composite materials. Accordingly, the general objects of the present invention are to provide novel and improved methods and materials of such character.
2. Description of the Prior Art
Fiber-reinforced composite materials have attracted considerable interest in recent years. Such interest has been particularly strong within the aerospace industry where technological advances are becoming ever more dependent upon the development of light weight metal composites of exceptional strength. Composite metallic structures, which are reinforced with high strength, high modulous filaments or fibers having a high length-to-diameter ratio, have been demonstrated to have high specific properties.
With particular respect to the aerospace industry, titanium-based composites have been considered for high temperature applications because of the high-temperature strength and low density of titanium and its alloys. Fiber-reinforced titanium-based composites, if available, would exhibit increased temperature capability; improved shear, transverse, and off-axis properties; and better erosive environment durability compared with presently available aluminum matrix and polymeric matrix composite systems.
Returning, briefly, to a general discussion of fiber-reinforced materials, the efficiency of transfer of tensile stress from a matrix to a filament within the matrix depends upon the integrity of the bond between the filament and the matrix material. Assuming a good bond, optimum strength of the composite material will be achieved if the major portion of an applied load is carried by the reinforcing fibers. In order for this to occur, the fibers must be strong, have a high length-to-diameter ratio and must be properly oriented with regard to the direction of the applied load. Because of its commercial availability, strength and desirable aspect ratio; i.e., a high length-to-diameter; boron filaments have attracted considerable attention for use as reinforcing fibers. Commercially available boron fibers are, in fact, tungsten filaments which have been coated with boron by means of a continuous vapor deposition process.
Previous attempts to fabricate boron fiber reinforced titanium alloy matrix composite materials have met with only limited success. In order to provide a usable product, sheets of the matrix material and layers of the reinforcing fibers are stacked so that the top of each reinforcing fiber is positioned opposite the bottom of a superimposed metal sheet. The stacked layers are laminated, typically by a vacuum hot pressing operation, into an integrally bonded composite structure which can thereafter be machined into the desired form. It has been established that, at consolidation temperatures sufficiently high to promote bonding of titanium matrix material, layer to layer within the stack, an interfacial reaction occurs between boron fibers and the matrix resulting in the formation of a layer of intermetallic compound. Fracture events within the plurality of brittle layers of intermetallic compound which occur throughout the laminate have limited the strain capability and thus the strength of previously available boron titanium composite materials.